The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not admitted to describe or constitute prior art to the present invention.
Listeria monocytogenes (Lm) is a facultative intracellular bacterium characterized by its ability to induce a profound innate immune response that leads to robust and highly functional CD4 and CD8 T cell immunity specific for vaccine-encoded Ags. Lm is a food-borne bacterium with increased pathogenicity among immune compromised individuals, including patients with cancer or other viral-induced immune deficiencies, pregnant women, the elderly and infants.
Recombinantly modified Lm vaccine platforms engineered to encode a designated antigen(s) relevant to a selected targeted pathogenic agent or malignancy have formed the basis for several human clinical trials. As Listeria can be a pathogenic organism, and particularly in the immunocompromised, it is preferred that the administration step comprises administering an attenuated Listeria that encodes an expressible, immunologically active portion of an antigen of interest. “Attenuation” refers to a process by which a bacterium is modified to lessen or eliminate its pathogenicity, but retains its ability to act as a prophylactic or therapeutic for the disease of interest. By way of example, genetically defined live-attenuated Lm ΔactAΔinlB, which is deleted of two virulence genes and is attenuated >3 logs in the mouse listeriosis model, retains its immunologic potency and has been shown to induce robust CD4 and CD8 T cell immunity in both mouse models of human disease as well as in humans, and has been shown to be safe and well-tolerated in clinical settings among patients with various solid tumor malignancies.
Listeria strains have been most commonly engineered to secrete a tumor antigen as a fusion with all or a portion of a secreted Listerial protein, such as listeriolysin O (LLO) or ActA. It has been suggested that a possible reason for the efficacy of such vaccine constructs may be the presence of amino acid sequences within LLO and ActA called “PEST” motifs. PEST regions (P, proline; E, glutamic acid; S, serine; T, threonine) are hydrophilic amino acid sequences that reside near the NH2 or COOH termini of certain proteins. They are thought to target proteins for rapid degradation by the cellular proteasome. To be recognized by T lymphocytes, protein antigens must be converted into short peptides bound to MHC molecules, which are displayed on the surface of antigen presenting cells. And, indeed, the PEST region of LLO has been suggested to be crucial to the success of Listerial vaccines, as the supply of peptides available for presentation by MHC class I molecules can be increased by shortening the cellular half-life of a protein.